For those who are new to iptables try firestarter
http://firestarter.sourceforge.net/

it is a GUI interface for IPTABLES. I am pretty new to Linux packet filtering firewalls but this made it really easy to understand conceptually.

Also make sure you read this tutorial on IP Masquerading if you have an internal LAN:
http://www.tldp.org/HOWTO/IP-Masquerade-HOWTO/


pEaCe